It is known to provide a MOSFET with on-die or internal temperature protection. For example, as described in Infineon Technologies' Application Note entitled “Temperature sense concept—Speed Tempfet®—Principle of the temperature sense concept of the Speed-TEMPFET family” by Benno Köppl, May 1999, a temperature sensing thyristor can be mounted on a MOSFET using chip-on-chip technology, the thyristor either being connected in parallel with the gate-source electrodes of the MOSFET within the device or having external connections. In the former case the thyristor fires to reduce the MOSFET gate-source voltage, thereby turning off the MOSFET, when its temperature exceeds a trip level; this has the disadvantage of requiring a series gate resistor, which limits the MOSFET switching speed. In the latter case, external circuitry, such as a microcontroller, driver IC (integrated circuit), or discrete circuitry, is required to monitor and respond to the thyristor state.
It is also known to integrate anti-parallel electrically isolated polysilicon diodes on a MOSFET die for temperature sensing, for example as described in Vishay Application Note AN820 entitled “Temperature Sensing Power MOSFET” by K. Pandya, 13 Jul. 2001, or International Rectifier device IRLBD59N04E Data Sheet, Nov. 13, 2001. With such devices, an external circuit is used to sense temperature based on the temperature-dependent forward voltage drop of the diodes.
These known arrangements require the use of specific MOSFET devices from respective manufacturers to provide the temperature sensing and any protection facilitated thereby. It is desirable to provide MOSFET junction temperature sensing in a manner that facilitates the use of a wide range of MOSFETs that do not incorporate such temperature sensing facilities.
One application of a power MOSFET is for switching a supply voltage to one or more switch mode power supplies which produce relatively low voltages for supply to electronic circuits. Typically, for example, the supply voltage may have a nominal value of 48V, and typically this is smoothed by a capacitance on the switched or output side of the power MOSFET. When the supply voltage is initially connected with the capacitance discharged, there is a substantial inrush current as the capacitance is charged from zero volts towards the supply voltage. At this time the inrush current is typically limited by controlling the MOSFET so that it is not fully enhanced (fully turned on to provide a low resistance). Conveniently the MOSFET gate voltage is controlled so that the MOSFET conducts a substantially constant drain current. Consequently, before the MOSFET is fully enhanced, the MOSFET is subject to a very high power dissipation.
For example, assuming a supply voltage of 50V and a constant current of 5 A for charging the capacitance, the MOSFET is subject to a power dissipation of 250 W during this turn-on period. The energy dissipated in the MOSFET may be tens of joules in a short period of for example 500 μs, depending on the charging current and the size of the capacitance. In contrast, a peak current of the order of 30 times higher would be required during subsequent operation, when the MOSFET is fully enhanced and has a low drain-source resistance, for a similar power dissipation in the MOSFET.
Accordingly, the junction or die of such a MOSFET is particularly prone to excessive temperature rise as a result of the inrush current, and it is under such circumstances that temperature sensing and limiting may be particularly likely to be required.
There is therefore a need to facilitate temperature sensing, including temperature limiting, of MOSFETs.